1. Field of the Invention
The invention relates to shaft bearing heat shields for electrodynamic machines, including electric induction motors. A specific exemplary application of the present invention is directed to bearing heat shields for totally enclosed fan cooled (TEFC) motors.
2. Description of the Prior Art
Induction motors generate heat as they convert electromagnetic energy to kinetic energy necessary to rotate the motor shaft under an applied load. Totally enclosed fan cooled (TEFC) motors have sealed housings. Heat is transferred from the internal rotor and stator to peripheral cooling fins on the motor housing. The motor drive shaft projects from one axial end of the motor. The other end of the shaft incorporates a cooling fan surrounded by a housing/shroud structure that directs air axially along the motor circumference, in order to increase convective heat transfer from the motor to atmosphere. While the driven fan assists heat transfer from the rotor and stator assemblies within the motor, a potential area for heat buildup is the drive side bearing opposite the driven fan. The drive side bearing has no direct access to driven fan cooling air.
In the past, one solution to provide cooling air to the drive bearing has been to construct the motor with another fan on the drive side. Such a construction adds additional length and complexity to the motor.
Another solution has been to add sealed cooling ducts and baffles within the motor housing and along its circumferential periphery in order to direct cooling air flow from the fan directly into the motor housing. This solution attempts to lower motor housing temperature in proximity to the drive side bearing. The drive side bearing, however, is still exposed to heat generated by the motor. In effect this solution attempts to introduce cooling air in proximity to the bearing housing, in order to enhance convective heat transfer away from the bearing. The airflow rate will vary as a function of the motor speed. When a motor under high load and RPM reduces speed it follows that the rate of convective airflow generated by the motor fan decreases precisely when the motor is in greater need of increased air flow to reduce heat capacitance in the motor housing. This increases the risk of unacceptable heat buildup in the drive side bearing housing.
Other induction motor designs, such as vertical shaft motors have constructed lubricant wells to provide a pool of oil surrounding the bearing and housing. Some vertical motors have incorporated auxiliary oil/air heat exchangers to assist oil cooling. Yet other hermetically enclosed motors have incorporated phase-changing chlorofluorocarbon liquid refrigerants sprays on bearing housings. However these cooling solutions are not practically applicable to horizontal shaft induction TEFC motors in typical industrial applications.
With respect to vented housing motors, other attempts have been made in the past to interpose sheet metal air deflectors in the internal motor cavity between the drive side motor bearing and the hotter rotor/stator assemblies so that cooling airflow in the cavity would carry heat away from the bearing housing region. Such shielding could also provide additional thermal resistance to convective heat transfer from the rotor/stator to the bearing. However, during continued motor operation such a shield eventually absorbs motor heat and it in turn radiates heat to the bearing housing. The single-layer partial shield/baffle in effect only provides transient thermal shielding to the bearing until the shield achieves operating temperature.
Thus, a need exists in the art for a motor bearing cooling solution that reduces heat transfer from the motor rotor and stator to the drive side bearing of TEFC motors and other types of electrodynamic machines.